DESCRIPTION. The goal of the proposed research is to understand how the generation of thrombin at the surface of platelets stabilizes blood clots. The primary roles of thrombin in clot formation are to catalyze fibrin polymerization and activate platelets. Which of these two mechanisms is most responsible for clot stability and under what flow conditions is unknown. The central hypothesis for the proposed research is that at venous shear stresses, stable clots can form in the absence of platelet activation but at arterial shear stress, platelet activation is the predominant mechanism for stabilizing clots. To study clot stability, we will use a novel microfluidic tool to introduce soluble factors into flowing blood. The rationale for this approach is that blood is a moving biological fluid in vivo, and therefore it is important to study blood phenotype in vitro under physiologically relevant flow conditions. The formation and morphology of the resultant clots will be measured by phase contrast, fluorescence, and electron microscopy methods. The stability of clot will be determined by analyzing its fragmentation under high shear stress. This innovative approach is expected to yield the following outcomes. In specific aim 1, we will determine how thrombin flux.affects fibrin polymerization, which is important because it will identify the amount of thrombin generation needed under flow to form a fibrin clot. In specific aim 2, we will decouple fibrin polymerization from platelet activation to determine the relative role of these two mechanisms in forming stable clots. By isolating the mechanism(s) that are responsible for clot stability at different flow conditions we expect to gain further insight into the pathology of bleeding disorders and embolism. LAY SUMMARY. The goal of this research is to understand the mechanisms responsible for forming stable blood clots. Poor clot stability is the result of bleeding disorders like hemophilia and the cause of stroke by emboli shedding from preexisting clots. By unraveling the mechanisms responsible for clot stability, this research will identify new therapeutic targets for individuals who suffer from these diseases. [unreadable] [unreadable] [unreadable]